1. Field of Invention
The present invention relates to a fuel cell, and more particularly to a method for fabricating a bi-polar plate of a fuel cell and the bi-polar plate thereof.
2. Related Art
The fuel cell, characterized in having high efficiency, quick reaction, silent operation, and low pollution, is widely applied in fields such as electricity, industry, transportation, space, and military. The fuel cell is a power generation device capable of continuously converting chemical energy into electric energy directly. When the fuel cell is working, a fuel gas (for example, H2) and a combustion supporting agent (for example, O2) are transferred to an anode and a cathode of the fuel cell respectively, and the chemical energy is converted into electric energy through oxidation and reduction reactions.
The structure of a conventional fuel cell is generally formed of an anode plate, a cathode plate, and a solid electrolyte membrane sandwiched therebetween, which is also called a single cell. However, in practice, a plurality of single cells is connected in series to obtain a large output voltage. The adjacent fuel cells share one electrode plate, so that the electrode plate serves as both an anode and a cathode of the two adjacent fuel cells, and is thus referred to as a bi-polar plate.
The bi-polar plate is mainly made of a material having high electrical conductivity such as graphite, composite carbon, and metal. In order to satisfy the demand for high power density, light weight, and flexibility of the fuel cell in the market, the bi-polar plate needs to be designed lighter and thinner. However, if the graphite plate or composite carbon plate is made very thin, problems including undesirable air-tightness and insufficient structural strength of the bi-polar plate may occur, or fractures during processing or use may be easily caused due to material characteristics of the graphite such as low hardness and poor ductility.
The metallic material is easily processed and suitable for mass production, and also has excellent electrical conductivity and thermal conductivity, so that a thin and light bi-polar plate can be obtained with such the material. However, in the working process of the fuel cell, due to the reaction of the fuel or oxidant, the surface of the metal bi-polar plate may be easily oxidized and corroded to cause deterioration of the performance of the fuel cell and shortening of its service life. If corrosion-resistant noble metal, such as Au or Pt, is used to fabricate the bi-polar plate, or a protection layer made of noble metal is plated on the surface of the metal bi-polar plate, the manufacturing cost becomes rather high.
Taking a bi-polar plate made of a corrosion-resistant metallic material such as stainless steel, Al, or Ti for example, a dense oxidation layer formed of oxides can be provided on a surface of the bi-polar plate to avoid corrosion on the surface of the bi-polar plate caused by the electrolyte. However, the oxidation layer may also raise the contact resistance of the surface, so that the capability of the bi-polar plate for conducting electrons is deteriorated, and a total output voltage of the fuel cell is reduced.
A bi-polar plate structure of a fuel cell is presented in U.S. Pat. No. 6,828,040 (referred to as U.S. Pat. No. 040 for short hereinafter). In this structure, a polymeric material is spray-coated on a stainless steel substrate and bonded to the substrate through pyrolysis. Electrically conductive graphite is added in the polymeric material to reach a content of at least 90% in total, thereby isolating the stainless steel substrate from the corrosion and oxidation effects of the ambient environment and acquiring electrically conductive characteristics at the same time.
However, in U.S. Pat. No. 040, as an electrically conductive protection film is formed by spray-coating and the solvent in the polymeric material is volatilized in the pyrolysis process, plenty of small holes are generated in the eventually formed electrically conductive protection film, resulting in an undesirable density of the protection film, such that the acid solution of the fuel cell may penetrate into the bi-polar plate through these small holes. Thereby, in U.S. Pat. No. 040, multiple layers of the polymeric material need to be coated on the surface of the stainless steel substrate, and several pyrolysis processes are required to avoid the above problem. However, in this case, the fabrication process of the bi-polar plate becomes too complicated and the manufacturing cost is largely increased.